Compile test framework using R2RTest (#42617)

Compile test framework using R2RTest (#42617)

This change switches crossgening of framework when building coreclr
tests to using R2RTest instead of explicitly invoking crossgen /
crossgen2.
To support that, the R2RTest was slightly updated to support specifying
target architecture, full path to crossgen2 and overriding the output
directory.